The present invention relates to a device and a method for generating a clock signal.
Conventionally, information is transmitted from a sender (a signal source) across a channel to a receiver at a different location, whereby analogues and digital data is transferred. Therefore, the signal source varies a physical parameter (in particular the electric voltage or the frequency of electromagnetic waves) and the receiver measures this parameter. The wanted signal delivered by the signal source is always superimposed by unwanted noise, since thermal noise occurs in all electronic circuits due to the thermal movement of charge carriers.
Bandpass filters are being utilized in order to suppress the noise and in order to achieve a very good signal/noise ratio. A bandpass filter is introduced into the signal channel in order to let pass frequency component within a frequency band having a bandwidth B=f2−f1 and in order to block or substantially attenuate the remaining frequency components. F1 is the lower limiting frequency and f2 is the upper limiting frequency. F0 is the medium frequency or resonant frequency of the bandpass filter. The wanted analogues or digital signal without noise includes only frequency components within the bandwidth of the bandpass filter. The carrier frequency of the transmitted signal corresponds to the medium frequency of the filter.
Quite often the problem occurs that a periodic clock signal must be generated for a given transmission channel, wherein the frequency of the clock signal is correlated with the carrier frequency of the wanted signal. In particular, this is necessary if a time dependent processing of the received signal must be performed, for example for automatic gain control. The clock recovery in digital transmission technology has the goal of determining the transmission clock of the sender from the received digital signal and thereby making it possible to sample the received signal exactly. The receiver must perform the clock recovery in order to determine the periodic sampling points in time of the data stream. It is impossible to analyse the received digital signal correctly without knowledge of the exact time alignment of the signal.
Conventionally phase looked loops are used for clock recovery purposes. They include a phase detector and a voltage controlled oscillator. The oscillator generates the clock signal depending on the output voltage of the phase detector. These circuits have the disadvantage that they are sensitive to temperature and process variations. Consequently, the recovered clock may not be correlated to the transmitted signal anymore.
For these and other reasons, there is a need for the present invention.